In general, preparation of plate glass by a float glass process has been carried out as follows: Molten glass is continuously flowed into a bath with molten metal (typically molten tin) filled therein. The molten glass is adjusted to have a certain width, flowing in an advancing direction. For example, when a float glass process is used to prepare plate glass having a thickness not more than a so-called equilibrium thickness, molten glass is supplied on a molten metal bath, and then the molten glass flow is stretched on the bath by applying a pulling force to the molten glass in the advancing direction thereof. Since the molten glass flow attenuates in a width direction thereof when the molten glass flow is stretched, the molten glass flow has opposite edges engaged with top rollers to restrict the attenuation.
However, the molten glass flow is required to have a certain extent of viscosity for effective engagement with the top rolls. In case of e.g. soda lime silicate glass, the molten glass flow is required to have a temperature of about 750.degree.-about 950.degree. C. and a viscosity of about 10.sup.4 poise-10.sup.7 poise at the time of engagement with the top rollers. Such a viscosity creates a problem in that a glass surface is likely to be subjected to minute wave, i.e. distortion by engagement with the top rolls.
When the molten glass flow has the opposite side edges held by engagement with the top rolls, the molten glass flow is cooled by the top rolls at portions thereof where the top rolls are engaged. As a result, the viscosity of the molten glass lowers at such portions in comparison with a central portion of the molten glass to prevent plate glass from having a target thickness. These portions can not be used for commercial products, which causes a decrease in production yield.